Pocket Mites

Mites or acari are small arachnids, close to spiders and scorpions, but much more numerous in species and in individuals (Baker and Wharton, 1952). They are to be found in almost every habitat available to animal life. Some are free in the soil, forest litter, on plants, in fresh or sea waters. Others are parasitic or symbiotic on plants and animals or transmit diseases like tsutsugamushi, the scrub typhus etc. Some are floating in the air, with air currents, with the dust, and can cause allergies. Their variety is immense, perhaps millions, and a large number of them remain to be discovered and described, mostly in tropical forests and in the soil. In achieving biodiversity they have surpassed even their cousins, the insects. Ticks, which are big mites, are mostly blood suckers, and get inflated after a blood meal. They transmit diseases to man and domestic animals, like piroplasmosis and many bacterial and viral diseases. Lyme fever, caused by a spirochaete, is an important disease, and, in the US, deer are the main reservoir of the pathogen. The Erythraeidae are reddish, predacious mites, with legs adapted for running. Dinothrombium, the dinosaurs of mites, are very big mites running on the ground in the African tropics. They look like big reddish silken balls continuously agitated on wet grounds.

Mites, though having generally eight legs in the adult stage, have many things in common, mostly in biology, with the insects. For instance, they have six legs in the larval instars, which situation has led to some serious errors; for instance, during a congress of entomology, a specialist described a new order of insects, which turned out to be larval mites.

Among mites, there are some blind species, like Amblyserus; they have an extra-ocular vision, the receptors for which could not yet be located.

Mites are close to Solifugae, the camel-spiders, a kind of big arachnids, quite disgusting in appearance, and have two pairs of forceps (chelicerae). They are efficient predators, but quite harmless to man. They look like insects and walk on six legs, as the first pair of legs act as feelers and grab food. They are common in America, North Africa and Middle East. They fight and eat insects, rodents, lizards, small snakes, and even small birds. Their bite is ferocious. They tear their victim's integument and regurgitate a digestive liquid, which liquefies the organs of the prey, and then they suck up the liquid contents into their stomach. Female Solifugae are good mothers and they protect their eggs and their young ones, but the offspring are also ferocious and young ones are often cannibals. The Solifugae did cleaning of the house of PJ in Ethiopia, and in the morning not a vermin remained in the rooms. They were coming from outside under the doors and were leaving in the morning, having accomplished their work. They were the famous Galeodes arabs.

Mite life style is so complex that a book of more than even 1000 pages will not adequately cover it. They live as parasites in the lungs of birds and reptiles, in bee tracheae, and inside the skin of mammals. They are responsible for human scabies and for the galls in plants. Demodex folliculorum is a mite parasitic in the bases of human and animal hairs.

What is extraordinary, and relatively little known, is that several living beings (plants, lizards, bugs, bees, wasps, moths and probably some others) have acquired, during their evolution, special pockets to lodge the mites. In fact, as plant galls have evolved to limit the damage done to the plant body by insects and other gall producing organisms, it seems that these pockets in animals have evolved in order to neutralize the parasite presence. In several cases, it appears that the mites have occupied preexisting natural cavities (glands, typanic organs, cavities in plant stems etc.). In exchange of the lodging, and sometimes of food, plant mites are useful to their host. They prevent epiphyte growth, which affect photosynthesis efficiency of the host. They also serve to keep the leaf laminae of the host clean.

The transportation of mites, scorpions, insects and even plants by insects or arthropods is a phenomenon called "phoresy", provided it is only transportation and no parasitism. In the cases, we have cited above, the transportation is life long for the animal host, and, therefore, we may use the term of ectoparasitism or epizoitic symbiosis for such cases. For an example of such an association see the chapter "Forest on back".

Very distinct from insect, mite and bacterial galls (cecidia) are acaro-domatia, which are gall like swellings lodging small mites, called pocket mites. The acarodomatia or pockets lodging mites preexist the entry of pocket mites into them in many plants. It has been suggested that the plant genome induces formation of acarodomatia to invite mites. A similar symbiotic association is known between ants and plants. Plants develop cavities and crevices (myrmecodomatia) to lodge and invite ants. (For this symbiotic association between plants and ants see the chapter "Omnipresent ants".) It has sometimes been doubted that the ant or mite lodgings are purposely genome guided. But it cannot be doubted that such associations are symbiotic, that is useful to the plant as well as to the ants/mites. Feeding habits of mites associated with plants has been detailed by Evans et al. (1961).

Acarocecidia should be carefully distinguished from acarodomatia, which preexist the entry of mites. Acarocecidia, on the other hand, are developed after entry into the plant body; they are in fact mite induced galls.

The mite pouches are mostly found along the axis of the leaf veins of certain plants. They are known even in 42 million-year-old fossils in Australia (O'Dowd et a/.,1991; O'Dowd and Willson, 1991; O'Dowd and Willson, 1991-1992). It seems that the plant associated mites (the herbivorous ones) clean the leaf surface by removing epiphylls, fungi, lichens, algae, spores, pollen etc., and carnivorous ones by eating the herbivorous mites, insect eggs, small insects. Many mites are serious agricultural pests. Some are free, i.e. not exactly agricultural pests. But many of this last mentioned category induce the formation of acaroce-cidia and are not lodged in acarodomatia. The pocket mites seem to be really efficient in cleaning the leaves and increasing the photosynthesis efficiency. In the tropics, organic growth on the leaf surface is a serious problem, and the pocket mites solve this problem effectively.

Similar pockets exist on bodies of wasps, bees, lizards and moths, and also in certain bugs. In the last case, there are secretory glands regularly invaded by mites. Are those acari really cleaners of these pockets? It has not yet been proven. The best explanation, that we can give at present, is that those pockets diminish the damages to the host body by keeping the mites in a specially conditioned place. In view of what we know about plant galls, this explanation seems to be acceptable. Let us now learn more about such pockets in animal bodies.

Professor Fain, in Belgium, has described (1970) a new genus of acari parasitic in the vestibules of a pair odoriferous glands of certain Coreidae bugs (Coreitarsonemus). Those vestibules were full of small whitish tiny bodies, which were mites of the family Tarsonemidae. It seems that this parasite is very common among bugs of the family Coreidae. The glands, infested with the mites, certainly feed the mites and fix them at this precise place in the body of the host.

PJ was once in Southern Korea when an epidemic of conjunctivitis, due to a virus, struck the country. All the Koreans, one after another, carried a bandage over one eye, then over the other, while doctors appeared vexed. When PJ questioned one of them: "Why covering over only one eye?", he answered, not without logic, "Because if we were covering both of them, we could not see anything". It is exactly what is going on with some night flying American moths. Mites attack and paralyze one tympanic organ, and always one and never both (Coineau and Kresling, 1974; Treat, 1975, 1983). If the two organs were attacked, the insect would be completely deaf and it would be in danger, and with it the parasitic mites. Tympanic organs in the moth are necessary to avoid radar using bats. Normally this mite, studied by Aslar Treat, settles in only one "ear", which can contain one hundred individuals, and not the other, and this situation ensures preservation of both the species (mite and moth). Either species has to choose the lesser of two evils. The moth can survive with only one ear, and with it the acari.

Among the females of certain Indo-Malaysian bees, close to our Xylocopa (the big blue bee of our areas), belonging to the genus Koptorthosoma, have mite pockets between the posterior part of the thorax and the base of the abdomen. Many ectoparasitic or phoretic mites are found in these pockets, and generally the mites live upon the females only. The males don't go back to the nest after copulating, which takes place always outside the nest. Is this the reason why they do not have mites? Answer to this question we do not know. Among the males of these Indo-malaysian bees, the mite pocket exists, but is always empty and remains in a rudimentary stage. Among the females of the Odynerus wasps from Arizona and Mexico also there is a cavity between the first and the second abdominal segments, and the cavity in females is full of small acari which do not interfere with the movements of the abdominal segments. Odynerus males do not have this pocket and do not carry mites. Mites are also present, as phoretic, among sweat bees (Halictidae) and on many other Hymenoptera (Eickwort, 1979).

Many lizards, iguanas, chameleons, geckos, possess small skin invaginations. Those invaginations or folds are situated on the neck, in the groin, in the postfemoral region etc., and are sometimes used in taxonomy. Those pockets contain acarian larvae, mostly Trombiculidae, and are met with among members of five lizard families (Arnold, 1986).

This phenomenon seems linked with the humid tropics. These pockets are already present in lizard embryos; this fact suggests that they are genetically fixed. Similarly, the callosities of the ostriches, and of the warthogs in Africa are also genetically fixed (Baldwin effect). These preexisting pockets in lizards are colonised by mites very early. The epidermis of these pockets is adapted to mite bites and repairs itself quickly. The mites are protected and, at the same time, the host minimizes the damage, which would be extensive if the mites spread all over the body.

A special mite family was discovered in the cloaca of two aquatic North American tortoises (Camin et al, 1967). This mite family, Cloacaridae, was described in 1967. They are very much modified morphologically and specially adapted to their strange habitat. They have been referred to as turtle crabs, and their transmission occurs venereally. Perhaps they are there in all tropical tortoises.

In conclusion it may be said that mite pockets exist in many plants and animals. These infoldings develop early, they preexist, and are colonised by mites at the earliest opportunity. Hence they seem to develop under influence of heredity. Among Hymenoptera the pocket mites infest only females. Evolution of these pockets seems to be an adaptation to minimize damage to the body by the parasitic mites. As the British say: "they keep the mites out of mischief". About association of mites with plants, it is generally accepted that it is symbiotic.

As the pocket mites are studied further, it is likely that we come to discover many more interesting facts. For example, some mites of the family Canestriniidae and some others perhaps show an inverted copulation pattern. The females penetrates the male to pick up the sperm, as among some rare butterflies and beetles.